1. Field of the Invention
This invention relates generally to DC power supplies and more specifically for discharging a bulk filter capacitor to a safe charge level after cessation of current supplied from an external power source.
2. Description of the Related Art
Computer systems are information handling systems which can be designed to give independent computing power to one user or a plurality of users. Computer systems may be found in many forms including, for example, mainframes, minicomputers, workstations, servers, personal computers, internet terminals, notebooks, and embedded systems. Personal computer (PC) systems, such as the International Business Machines (IBM) compatible PC systems, include desk top, floor standing, or portable versions. A typical PC system is a microcomputer that includes a system processor or microprocessor, associated memory and control logic, and a number of peripheral devices that provide input and output for the system. Such peripheral devices often include display monitors, keyboards, mouse-type input devices, floppy and hard disk drives, CD-ROM drives, and printers. The number of devices being added to personal computer systems continues to grow. For example, many computer systems also include network capability, terminal devices, modems, sound devices, voice recognition devices, electronic pen devices, and mass storage devices such as tape drives, CD-ROM drives, or DVDs.
Typically, computer systems are powered by a power supply that receives and converts alternating current (AC) power to direct current (DC) power that is used to power the computer system components such as the system processor. One type of AC to DC power supply used to supply current at DC voltages, converts power from an AC power source such as 120 V, 60 Hz power from a wall outlet. This is accomplished by rectifying the voltage level of an AC power source to an unregulated bulk DC voltage. This unregulated DC voltage typically has a ripple waveform component. To "smooth" out the ripple component, most power supplies incorporate a bulk filter capacitor or bulk reservoir capacitor. Typically, a bulk filter capacitor stores charge during the ripple peaks and releases charge during the low portion of the ripple cycle. Also, AC to DC power supplies typically include a DC to DC converter (also known as a regulator) for providing DC power to the computer system within specified tolerances.
Because bulk filter capacitors can store a large amount of charge at relatively high DC voltages such as at 400 VDC and higher, they must be discharged when the AC power is removed or disconnected from the power supply in order to comply with safety standards to prevent the bulk filter capacitor from becoming a shock hazard. The IEC 950 standard requires the bulk filter capacitor to be discharged to a voltage level of less than 2 VDC. One method of discharging the bulk filter capacitor when the AC power source is disconnected and AC current ceases to be supplied to the power supply is to connect a bleeder resistor in parallel with the bulk filter capacitor. However, a bleeder resistor continually dissipates power during the normal operation of the power supply resulting in wasted energy which appears as heat in the power supply.
To remedy the problem of wasted energy from a continuously connected bleeder resistor, some systems implement a special switch that is activated to electrically couple the bulk filter capacitor to a resistive element to discharge the bulk filter capacitor. However, these circuits generally require a separate switching element that is robust enough to handle the discharge current.
What is needed is a power supply that discharges a bulk filter capacitor in response to the removal of an external power source and accomplishes this objective with a minimal number of components.